1. Field of the Invention
The present invention relates to pressure equalising devices used in oil and gas wells and in particular to improved actuation methods and apparatus for pressure equalising devices. In one of its aspects the invention relates to an improved wellbore plug.
2. Description of Related Art
During the lifetime of an oil/gas production well, various servicing operations will be carried out to the well to ensure that the efficiency and integrity of the well is maximised. These include a full work over, a surface wellhead tree change, side tracking or close proximity drilling operations. To allow any of these operations to be done safely and to accommodate verification pressure tests from surface, it is necessary to install a plug (or plugs) into the production tubing to create a barrier to test against and provide isolation from the production zones.
These plugs are typically run into or retrieved from the wellbore on wireline or tubing strings. When retrieving plugs it is necessary to equalise pressure above and below prior to unlocking and removal. This often involves an extra intervention run to initiate pressure equalisation prior to retrieval.
Various types of pressure equalising devices have been developed for use with plugs, including those known as “pump open plugs” and a “pressure cycle plugs”. Recently, there has been proposed a pressure equalising device with a controlled timed release actuation, as disclosed in WO 2005/052302. This device is configured to open the plug such that there is fluid communication through the plug to the upper and lower portions of the wellbore, in response to an applied and maintained pressure within a predetermined pressure range (or “opening window”) for a certain period of time. If this condition is not satisfied, the device is not actuated. This enables a range of different pressure tests to be performed in the wellbore, for example at pressures outside of the predetermined range and/or at pressures within the opening window but over a time period shorter than that required for opening.
This plug operates on the principle that pressure testing events do not occur for long durations at pressures within the predetermined pressure zone. Conversely, an actuating pressure event for opening the plug must be identified as being in the predetermined zone for a sufficient period of time within a defined pressure zone.
One particular problem associated with the prior art devices is that they operate on the principle of applied differential pressures, requiring knowledge of the pressure in the wellbore. That is, the pressure applied at surface must correspond to the pressure suitable for actuating the equalising device. In many wells it is common for changes in the well and formation environment to affect the pressure of fluid in the wellbore. Thus, it might be necessary to adjust the applied pressure to account for any variation in the ambient wellbore pressure. These pressure variations can prevent a user from knowing what applied pressure is adequate to satisfy the conditions necessary for opening the plug.
In addition, gradual increases in wellbore pressure due to environmental conditions might also lead to the wellbore pressure falling within the predetermined zone for a period of time such that there is a risk of unintentional actuation of the equalising device.